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A Tale of Three Mysterious Spectral Features in Carbon-Rich Evolved Stars: The 21 Micrometer, 30 Micrometer, and "Unidentified Infrared" Emission Features

The mysterious "21 micrometer" emission feature seen almost exclusively in the short-lived protoplanetary nebula (PPN) phase of stellar evolution remains unidentified since its discovery two decades ago. This feature is always accompanied by the equally mysterious, unidentified "30 micrometer" feature and the so-called "unidentified infrared" (UIR) features at 3.3, 6.2, 7.7, 8.6, and 11.3 micrometer which are generally attributed to polycyclic aromatic hydrocarbon (PAH) molecules. The 30 micrometer feature is commonly observed in all stages of stellar evolution from the asymptotic giant branch (AGB) through PPN to the planetary nebula phase. We explore the interrelations among the mysterious 21 micrometer, 30 micrometer, and UIR features in the Galactic and Magellanic Cloud of the 21 micrometer sources. We derive the fluxes emitted in the observed UIR, 21 micrometer, and 30 micrometer features from published ISO or Spitzer/IRS spectra. We find that none of these spectral features correlate with each other. This argues against a common carrier (e.g., thiourea) for both the 21 micrometer feature and the 30 micrometer feature (otherwise these two features should correlate). This also does not support large PAH clusters as a possible carrier for the 21 micrometer feature.